U.S. Pat. No. 4,711,746 issued Dec. 8, 1987 to Drader is directed at a process, and an associated device, for welding thermoplastic by high pressure injection of molten plastic into the area to be welded. Although the process has been found to be relatively efficient and effective, the disclosed device tends to be quite expensive to manufacture and maintain.
In particular, the Drader device comprises a heated barrel having an internal chamber and a nozzle forming a restrictive orifice outlet. Plastic filler rod is forced into the chamber by drive rolls in order to generate a high pressure within the internal chamber of about 2500 psi. The molten plastic is thus forcefully ejected from the internal chamber through the orifice outlet. The drive rolls are actuated by an air driven ratchet wrench having an output shaft which is connected to a worm, which in turn meshes with a worm gear. The worm gear is connected to a first drive roll, which drivingly engages a second drive roll such that the drive rolls rotate together. The rod is gripped between the drive rolls and advanced towards the internal chamber through an inlet of the barrel.
Thus, Drader discloses a complex feed system for the rod, which tends to render the device relatively uneconomical to both produce and maintain. Further, given the specific configuration of the drive rolls, a spaced distance or gap exists between the inlet of the barrel and the location at which the rod is gripped between the drive rolls. As a result, the rod is not substantially supported between the inlet and the drive rolls, which may result in buckling of the rod when generating a high pressure in the internal chamber.
As well, although the system of gears and drive rolls in Drader may inadvertently inhibit movement of the rod rearward away from the barrel inlet, to a certain extent, the drive rolls and gears are not specifically designed for this function and may therefore provide a relatively inefficient mechanism for accomplishing this task. A separate or distinct means or mechanism for inhibiting this rearward movement of the rod is neither discussed nor disclosed.
Further devices for dispensing an adhesive are disclosed by U.S. Pat. No. 3,774,817 issued Nov. 27, 1973 to Whittaker and U.S. Pat. No. 4,379,516 issued Apr. 12, 1983 to Barlogis. Neither of these patents specifically discuss or disclose the use of their respective devices for high pressure injection of thermoplastics. Furthermore, each of these patents discloses a relatively complex device, which is unsuitable for injecting a molten thermoplastic under a relatively high pressure given the specific structure of the device.
Whitaker discloses a feed mechanism for advancing a rod of adhesive from a guide tube into a premelt canal section, which communicates with a heated reservoir. The feed mechanism comprises a gripping assembly and a complex pneumatic assembly, which apply a force to the rod at the location of the guide tube in order to advance the rod forwards into the premelt canal. The guide tube and the premelt canal are spaced apart to provide an "air gap" therebetween for dissipation of heat from the premelt canal and for insulation of the guide tube. As a result, the rod is unsupported between the guide tube and the premelt canal, which may result in buckling of the rod in the event a force is applied to the guide tube sufficient to generate a high pressure within the heated reservoir. Furthermore, the device attempts to avoid the generation of high pressures by specifically allowing for a slight rearward or return movement of the rod following its forward advancement into the premelt canal. This rearward movement is allowed to relieve pressure in the premelt canal and thereby prevent the flow of molten adhesive out of the air gap.
Apart from the permitted rearward movement, any tendency of the rod to move rearwards in Whittaker, away from the premelt canal, is inhibited by a finger stop. The finger stop provides a slender projection which passes through a slot in the guide tube for engaging the rod. A single sharpened end or point of the projection engages the rod at a single point on the circumference of the rod such that only a limited force may be applied by the finger stop to inhibit the rearward movement. The finger stop may therefore provide a relatively inefficient and ineffective mechanism for inhibiting rearward movement of the rod in the event a relatively strong rearward force is applied to the rod.
Barlogis similarly discloses a device for advancing an adhesive rod forwards to a heated chamber by the application of a force to the rod. The force may be applied manually, pneumatically or by a camming mechanism. In each of these cases, the rod is not substantially supported between the location of the application of the force and the heated chamber, which may result in buckling of the rod in the event a high pressure is sought to be generated in the heated chamber. Furthermore, the device specifically attempts to avoid the generation of undesirable high pressures in the heated chamber by providing a biased return element which brings the rod rearward when the forward force is no longer exerted in order to avoid any untimely dispensing of the adhesive through the heated chamber.
Finally, U.S. Pat. No. 4,513,886 issued Apr. 30, 1985 to Price discloses an apparatus for feeding an adhesive rod towards a melt chamber. This patent does not specifically discuss or disclose the use of the apparatus for high pressure injection of a thermoplastic. Furthermore, this patent discloses a relatively complex device, which may render the device relatively uneconomical to produce and maintain.
In particular, Price discloses a fixed tube which communicates with the melt chamber. A guide tube slidably fits within the fixed tube. Further, the guide tube is secured to a lead-in tube which is clamped inside a support tube. The rod extends from an upstream end through the lead-in tube, into the guide tube and then into the fixed tube to a downstream end. The feed mechanism comprises a feed jaw which projects through slots in the support and lead-in tubes for engagement with the rod. To advance the rod, the jaw engages the rod and the jaw, along with the support, lead-in and guide tubes, are advanced forward such that the guide tube passes inside the fixed tube.
Upon retraction, a retaining jaw grips the rod to prevent its return movement with the feed mechanism. The retaining jaw projects through slots in the fixed tube for engagement with the rod. As a result, the jaw inhibits rearward movement by engagement of the rod at a single point or defined location on the rod such that only a limited force may be applied by the jaw. The retaining jaw may therefore provide a relatively inefficient and ineffective mechanism for inhibiting rearward movement of the rod in the event a relatively strong rearward force is applied to the rod.
Further, the retaining jaw of Price is located adjacent the downstream end of the rod, while the feed jaw is located adjacent the upstream end of the rod such that the rod is pushed through the retaining jaw by the feed jaw. As a result, if the retaining jaw fails to release the rod upon advancement of the feed jaw towards the melt chamber, the feed mechanism will compress the rod against the retaining jaw, which is likely to cause damage to the internal structure of the device. The greater the force applied to advance the rod, the greater the likelihood of resulting damage to the structure of the device.
There is therefore a need in the industry for an improved apparatus for use in welding thermoplastics which is relatively simple in design such that it may be economically manufactured and maintained, as compared to similar known apparatuses. More particularly, there is a need for an apparatus for advancing a rod comprised of a thermoplastic material to a heated chamber for injection therefrom under pressure. Further, there is a need for the apparatus to be capable of applying an advancement force to the rod in a first direction towards the heated chamber which is sufficient to provide a relatively high injection pressure within the heated chamber, as compared to known devices for performing a similar function.
In addition, there is a need in the industry for a retaining mechanism for the improved apparatus. In particular, there is a need for an improved retaining mechanism which inhibits the movement of the rod in a second direction opposing the first direction, which is relatively simple in its structure as compared to known retaining mechanisms.